differenceOrd function
============ Differencing ============
Implementation
List<double> differenceOrd(List<double> x, int d){
if (d==0) return List<double>.from(x);
var y=List<double>.from(x);
for (int k=0;k<d;k++){ final ny=<double>[]; for (int i=1;i<y.length;i++) ny.add(y[i]-y[i-1]); y=ny; }
return y;
}